Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of cellular division, DNA replication, genetic variation, asexual reproductive strategies, angiosperm fertilization, human reproductive anatomy, embryonic development, and reproductive health. Chapter 7 of Class 10 Biology, “How do Organisms Reproduce?”, explores the biological mechanisms that ensure the continuity of life across generations. It examines how asexual reproduction (fission, budding, fragmentation, regeneration, spore formation, and vegetative propagation) allows rapid multiplication; investigates how sexual reproduction introduces genetic variation through the fusion of haploid gametes; and details the physiology of human puberty, the menstrual cycle, placental nourishment, and modern contraceptive methods. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all three in-text question sets (Pages 128, 133, and 140) to the complete chapter-end exercises (Questions 1 to 11 on Page 141)—has been solved with exhaustive detail. Short-answer conceptual responses follow the official 30–40 word limit, while 3-mark and 5-mark physiological comparisons use structured tables and clear biological schematics. Key scoring terms, official CBSE board year tags, and comparative charts have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. Modes of Asexual Reproduction in Organisms
| Asexual Method | Defining Characteristic / Mechanism | Example Organism(s) |
|---|---|---|
| Binary Fission | Parent cell divides into two identical daughter cells in any plane (or specific longitudinal plane). | Amoeba (irregular plane), Leishmania (longitudinal plane). |
| Multiple Fission | Parent cell divides into multiple daughter cells simultaneously inside a protective cyst during unfavorable conditions. | Plasmodium (Malarial parasite). |
| Budding | A small outgrowth (bud) develops due to repeated cell division at one specific site, matures, and detaches. | Hydra, Yeast. |
| Fragmentation | Multicellular organism with simple organization breaks into smaller pieces upon maturation, each growing into a new body. | Spirogyra (Green filament alga). |
| Regeneration | Specialized regenerative cells proliferate to reconstruct a complete organism from severed body parts. | Planaria, Hydra. |
| Spore Formation | Blobs on sticks (sporangia) produce thousands of microscopic, thick-walled spores that germinate in moisture. | Rhizopus (Bread mold), Mucor, Ferns. |
| Vegetative Propagation | New plants develop from vegetative parts (roots, stems, leaves) without seed formation. | Bryophyllum (leaf notches), Potato (tubers), Sugarcane (stem). |
2. Comparison: Asexual vs. Sexual Reproduction
| Parameter / Basis | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Number of Parents | Single parent involved; uniparental. | Two parents (Male and Female) involved; biparental. |
| Gamete Formation | No gametes or specialized sex cells are formed. | Specialized haploid gametes (sperm and egg) are formed by meiosis. |
| Fertilization / Zygote | No fertilization occurs; no zygote is formed. | Fertilization (fusion of gametes) forms a diploid single-celled Zygote. |
| Genetic Variation | Offspring are genetically identical clones of the parent; negligible variation. | Offspring inherit genes from both parents; promotes high genetic variation and evolutionary adaptation. |
| Rate of Multiplication | Rapid and energy-efficient mode of reproduction. | Comparatively slower and energy-intensive process. |
3. Classification of Contraceptive Methods
| Contraceptive Method | Mechanism of Action | Examples / Devices | Protection Against STDs |
|---|---|---|---|
| 1. Mechanical Barrier | Physically blocks the entry of sperm into the female genital tract. | Condoms (male/female), Diaphragm, Cervical caps. | Yes, highly effective against STDs (HIV/AIDS, Syphilis). |
| 2. Chemical (Hormonal) | Changes the hormonal balance of the female body to prevent ovulation. | Oral Contraceptive Pills (e.g., Saheli, Mala-D). | No protection against STDs. |
| 3. Intrauterine Devices (IUDs) | Placed in the uterus to prevent implantation of the blastocyst. | Copper-T, Loop, Hormonal IUDs. | No protection against STDs. |
| 4. Surgical Methods | Permanently blocks gamete transport ducts by cutting and tying. | Vasectomy (Vas deferens tied in males), Tubectomy (Fallopian tubes tied in females). | No protection against STDs. |
NCERT In-Text Questions: Set 1 (Page No. 128)
Question 1 What is the importance of DNA copying in reproduction? [CBSE 2024, 2020]
Answer: DNA (Deoxyribonucleic acid) carries the master genetic blueprint that determines the structural and physiological body design of an organism:
- Transmission of Genetic Traits: DNA copying ensures that genetic information is faithfully transmitted from parents to offspring, preserving species characteristics.
- Creation of Cellular Apparatus: DNA replication is accompanied by the synthesis of additional cellular machinery, ensuring that both resulting daughter cells survive and function after division.
- Origin of Evolutionary Variation: Biochemical copying mechanisms are never 100% error-free. Small inaccuracies during replication introduce genetic variations, which serve as the raw material for natural selection and environmental adaptation.
Question 2 Why is variation beneficial to the species but not necessarily for the individual? [CBSE 2024, 2023]
Answer:
- For the Individual: A sudden genetic variation may disrupt an individual’s existing physiological balance or may not suit its immediate environment, resulting in premature death or poor survival.
- For the Species: Environmental conditions (temperature, water levels, chemical pollutants, global warming) can change drastically. If all individuals of a population were genetically identical clones, an extreme event could wipe out the entire population. However, if a few variants exist that can tolerate extreme conditions (e.g., heat-resistant bacteria surviving a sudden heat wave), they will survive, multiply, and prevent the species from becoming extinct.
NCERT In-Text Questions: Set 2 (Page No. 133)
Question 1 How does binary fission differ from multiple fission? [CBSE 2024, 2022]
Answer:
| Parameter | Binary Fission | Multiple Fission |
|---|---|---|
| Number of Offspring | Parent cell divides to form two identical daughter cells. | Parent nucleus divides repeatedly to form many daughter cells simultaneously. |
| Environmental Conditions | Occurs during favorable environmental conditions. | Occurs primarily during unfavorable conditions inside a protective cyst wall. |
| Division Plane | Nucleus divides once, followed immediately by cytoplasmic cleavage. | Nucleus undergoes multiple repeated divisions before cytoplasm divides. |
| Examples | Amoeba, Paramecium, Leishmania. | Plasmodium (Malarial parasite). |
Question 2 How will an organism be benefited if it reproduces through spores? [CBSE 2023, 2020]
Answer: Spore formation offers three major survival advantages to simple organisms like Rhizopus:
- Thick Protective Wall: Spores are enclosed in thick, resistant protective coats that allow them to survive extreme heat, desiccation, and adverse weather until favorable conditions return.
- Lightweight Aerial Dispersal: Spores are microscopic and light, enabling them to be easily dispersed by wind and water over vast geographic distances, preventing local overcrowding.
- Massive Numerical Yield: A single sporangium produces thousands of spores, ensuring a high probability of successful germination.
Question 3 Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration? [CBSE 2024, 2020]
Answer: Complex multicellular organisms (like humans, dogs, and birds) cannot reproduce through regeneration because:
- High Degree of Cellular Specialization: Their bodies are organized into complex tissues, specialized organs, and interconnected organ systems (nervous, circulatory, skeletal) with strict division of labor.
- Absence of Totipotent Cells: Complex animals lack unspecialized, totipotent regenerative stem cells throughout their adult tissues capable of multiplying, differentiating, and reconstructing an entire complex organism from a severed limb.
Question 4 Why is vegetative propagation practised for growing some types of plants? [CBSE 2024, 2022]
Answer: Vegetative propagation is widely practiced in horticulture and agriculture for the following reasons:
- Propagation of Seedless Varieties: It allows the multiplication of commercially valuable plants that have lost the capacity to produce viable seeds (e.g., banana, seedless grapes, rose, jasmine, orange).
- Genetic Uniformity (Clonal Purity): All offspring plants are genetically identical clones of the parent plant, preserving desirable traits like taste, flower color, disease resistance, and fruit yield.
- Faster Maturity and Yield: Plants raised through vegetative methods bear flowers and fruits much earlier than those grown from seeds.
Question 5 Why is DNA copying an essential part of the process of reproduction? [CBSE 2023]
Answer: DNA copying is essential because reproduction is fundamentally the transmission of biological information from parent to offspring:
- The DNA in cell nuclei contains the coded instructions for protein synthesis and body architecture.
- Without DNA replication, daughter cells would not receive the full complement of genetic material necessary to direct cellular metabolism, growth, and survival.
NCERT In-Text Questions: Set 3 (Page No. 140)
Question 1 How is the process of pollination different from fertilisation? [CBSE 2024, 2020]
Answer:
| Parameter | Pollination | Fertilisation |
|---|---|---|
| Definition | The physical transfer of pollen grains from the anther to the receptive stigma of a flower. | The biochemical fusion of a male gamete with a female gamete (egg cell) inside the ovule. |
| Site of Event | Occurs on the outer surface of the stigma (external floral event). | Occurs inside the embryo sac within the ovule of the ovary. |
| Requirement of Agents | Requires external pollinating agents (wind, water, insects, birds). | Requires no external pollinating agents; occurs via the growth of the pollen tube. |
| Resulting Product | Leads to germination of pollen grain and growth of pollen tube. | Results in the formation of a diploid single-celled Zygote, which develops into an embryo. |
Question 2 What is the role of the seminal vesicles and the prostate gland? [CBSE 2024, 2022]
Answer: The seminal vesicles and prostate gland are male accessory reproductive glands that secrete vital fluids along the vas deferens:
- Nutritional Support: Their alkaline secretions contain fructose, calcium, and proteins that provide essential nutrients to the swimming sperms.
- Medium for Motility: The fluid secretions make the transport of sperms easier by providing a fluid liquid medium (Semen).
- Neutralizing Acidity: The alkaline nature of the prostatic fluid neutralizes the acidic environment of the male urethra and the female vagina, protecting sperm viability.
Question 3 What are the changes seen in girls at the time of puberty? [CBSE 2023, 2020]
Answer: During puberty (between ages 10–12), under the influence of the female hormone Oestrogen, girls undergo the following secondary sexual changes:
- Development of Breasts: Breast size begins to increase with darkening of the skin around the nipples (areola).
- Growth of Body Hair: Appearance of pubic hair in the genital area and under the armpits.
- Onset of Menstruation (Menarche): Initiation of the monthly menstrual cycle, indicating ovarian maturation and egg release.
- Broadening of Pelvis: Widening of hips and deposition of subcutaneous fat around thighs and buttocks.
- Skin Changes: Increased activity of sweat and sebaceous glands, often leading to acne.
Question 4 How does the embryo get nourishment inside the mother’s body? [CBSE 2024, 2020]
Answer: The developing embryo gets nourishment from the mother’s blood through a specialized disc-like vascular tissue called the Placenta, which is embedded in the uterine wall:
- Villi Architecture: On the embryo’s side of the tissue, there are millions of microscopic finger-like projections called villi.
- Maternal Blood Spaces: On the mother’s side are large blood spaces surrounding the villi, providing a vast surface area.
- Nutrient & Waste Exchange: Glucose, amino acids, and oxygen diffuse directly from the mother’s blood across the villi into the embryo’s bloodstream, while carbon dioxide and metabolic nitrogenous wastes generated by the embryo diffuse into maternal blood for excretion.
Question 5 If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases? [CBSE 2024, 2023]
Answer: No, a copper-T will NOT protect her from sexually transmitted diseases (STDs).
A copper-T is an intrauterine device (IUD) placed inside the uterus solely to prevent pregnancy by releasing copper ions that suppress sperm motility and prevent implantation of the blastocyst. It does not create a physical barrier to block the exchange of bodily fluids or prevent the transmission of bacterial and viral pathogens (like HIV/AIDS, Gonorrhoea, Syphilis). Only barrier methods like condoms protect against STDs.
NCERT Chapter-End Exercises (Page No. 141)
Question 1 Asexual reproduction takes place through budding in: (a) amoeba (b) yeast (c) plasmodium (d) leishmania
Answer: (b) yeast Explanation: Yeast (a unicellular fungus) and Hydra reproduce asexually by developing a small outgrowth or bud that pinches off to form a new individual.
Question 2 Which of the following is not a part of the female reproductive system in human beings? (a) ovary (b) uterus (c) vas deferens (d) fallopian tube
Answer: (c) vas deferens Explanation: The vas deferens (sperm duct) is a component of the male reproductive system that carries sperms from the testes to the urethra.
Question 3 The anther contains: (a) sepals (b) ovules (c) pistil (d) pollen grains
Answer: (d) pollen grains Explanation: The anther is the male reproductive organ of a flower (part of the stamen) that produces and stores yellow pollen grains containing male gametes.
Question 4 What are the advantages of sexual reproduction over asexual reproduction? [CBSE 2024, 2020]
Answer: Sexual reproduction provides distinct evolutionary advantages:
- Creation of Genetic Diversity: It combines DNA from two distinct individuals, creating new and unique combinations of genes in every generation.
- Enhanced Survival in Changing Environments: Genetic variation provides species with the capacity to adapt and survive hostile environmental changes, diseases, and climate shifts.
- Driving Force of Evolution: It serves as the biological foundation for natural selection and speciation, whereas asexual reproduction produces identical clones with zero evolutionary plasticity.
Question 5 What are the functions performed by the testis in human beings? [CBSE 2024, 2022]
Answer: The testes (located outside the abdominal cavity in the scrotum) perform two dual functions:
- Spermatogenesis (Exocrine Function): Production of millions of male reproductive gametes (Sperms) at an optimal temperature of 2–2.5°C lower than normal body temperature.
- Endocrine Function: Secretion of the male sex hormone Testosterone, which controls the development of secondary sexual characteristics in boys during puberty and regulates sperm production.
Question 6 Why does menstruation occur? [CBSE 2024, 2023]
Answer: Menstruation occurs when the released egg is not fertilized by a sperm:
- Every month, one ovary releases a mature ovum, and the inner lining of the uterus (Endometrium) becomes thick, spongy, and richly vascularized with blood vessels to support a potential pregnancy.
- If fertilization does not take place, the unfertilized egg dies within 24 hours. The thick uterine lining is no longer required and slowly breaks down.
- The degenerated lining along with blood and mucus is discharged through the vagina as the menstrual flow (menstruation), lasting 3 to 5 days.
Question 7 Draw a labelled diagram of the longitudinal section of a flower. [CBSE 2024, 2020]
Answer: A complete flower consists of four concentric floral whorls:
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STRUCTURAL PARTS OF A COMPLETE FLOWER (L.S.):
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1. Non-Essential / Accessory Whorls:
• Calyx (Sepals): Outermost green leaves that protect the flower in bud stage.
• Corolla (Petals): Brightly colored, scented petals to attract pollinating insects.
2. Essential Reproductive Whorls:
• Stamen (Male Reproductive Unit / Androecium):
- Anther: Bilobed terminal sac containing yellow pollen grains.
- Filament: Slender stalk supporting the anther.
• Carpel / Pistil (Female Reproductive Unit / Gynoecium):
- Stigma: Sticky terminal landing platform for pollen grains.
- Style: Long, slender middle tube connecting stigma to ovary.
- Ovary: Swollen basal part containing ovules (each holding an egg cell).
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Question 8 What are the different methods of contraception? [CBSE 2024, 2022]
Answer: Contraceptive methods are categorized into four major approaches:
- Barrier Methods: Condoms, cervical caps, and diaphragms physically block sperm entry into the female reproductive tract (also prevents STDs).
- Chemical Methods (Hormonal Pills): Oral pills containing synthetic oestrogen and progesterone change hormonal balance to inhibit ovulation and egg release.
- Intrauterine Devices (IUDs): Copper-T or loops inserted by a doctor into the uterus prevent sperm motility and blastocyst implantation.
- Surgical Methods (Permanent):
- Vasectomy: Vas deferens in males is cut and tied to prevent sperm discharge in semen.
- Tubectomy: Fallopian tubes in females are cut and tied to block egg transit and fertilization.
Question 9 How are the modes for reproduction different in unicellular and multicellular organisms? [CBSE 2023]
Answer:
- In Unicellular Organisms: The entire body consists of a single cell. Reproduction occurs primarily through simple, direct asexual processes (binary fission, multiple fission, budding) where the single parent cell divides its nucleus and cytoplasm to form new independent organisms.
- In Multicellular Organisms: Organisms have specialized tissues, organs, and complex body designs. Simple cell division cannot recreate a complex body; therefore, they possess dedicated reproductive organ systems (producing male and female gametes) and reproduce predominantly through sexual reproduction.
Question 10 How does reproduction help in providing stability to populations of species? [CBSE 2024, 2020]
Answer: Reproduction ensures species population stability through two mechanisms:
- Replacement of Dying Individuals: In every biological niche, individuals die continuously due to aging, disease, predators, and natural hazards. Reproduction introduces new individuals to balance the death rate, maintaining a stable population size.
- Environmental Adaptation: The genetic variations generated during sexual reproduction allow members of a species to survive environmental fluctuations, ensuring the long-term survival of the species.
Question 11 What could be the reasons for adopting contraceptive methods? [CBSE 2024]
Answer: Adopting contraceptive methods is important for:
- Family Planning: Spacing children according to financial, physical, and emotional readiness.
- Maternal Health: Preventing frequent, unwanted pregnancies that severely deplete a mother’s nutritional, physical, and mental health.
- Prevention of STDs: Barrier methods (condoms) provide vital protection against life-threatening infections like HIV/AIDS, Gonorrhoea, and Syphilis.
- Population Control: Stabilizing national and global population growth to prevent resource depletion and poverty.
Frequently Asked Questions (FAQs) – Class 10 Biology Chapter 7
Question 1: What is the difference between self-pollination and cross-pollination? [CBSE 2024] Answer: Self-pollination is the transfer of pollen from the anther to the stigma of the same flower or another flower on the same plant; cross-pollination is the transfer of pollen to a flower on a different plant of the same species, promoting greater genetic variation.
Question 2: What is the function of the scrotum in males? [CBSE 2023] Answer: The scrotum is a pouch of skin hanging outside the abdominal cavity that maintains the temperature of the testes at 2 to 2.5°C lower than normal body core temperature, which is essential for viable sperm production.
Question 3: Where does fertilization take place in human females? [CBSE 2024, 2022] Answer: Fertilization takes place in the Ampulla of the Fallopian Tube (Oviduct) when a sperm fuses with the mature ovum.
Question 4: What is the gestation period in humans? [CBSE 2023] Answer: The human gestation period (the duration of embryonic development inside the mother’s uterus from fertilization to birth) is approximately 9 months (about 280 days or 40 weeks).
Question 5: What is tissue culture (Micropropagation)? [CBSE 2024] Answer: Tissue culture is a modern biotechnology technique where new plants are grown by removing small tissue samples from a plant growing tip and culturing them in a sterile, artificial nutrient medium rich in growth hormones to produce disease-free plantlets.
Question 6: What is the role of the pollen tube during plant fertilization? [CBSE 2022] Answer: Upon germination on the sticky stigma, the pollen grain grows a long pollen tube through the style to carry two male gametes safely into the ovule inside the ovary for double fertilization.
Question 7: What changes occur in a flower after fertilization? [CBSE 2024, 2020] Answer: After fertilization, the Zygote develops into an Embryo, the Ovule develops into a tough seed, the Ovary ripens into a Fruit, and the petals, sepals, stamens, style, and stigma shrivel and fall off.
Question 8: What are sexually transmitted diseases (STDs)? Give bacterial and viral examples. [CBSE 2024] Answer: STDs are infectious diseases transmitted through sexual contact:
- Bacterial STDs: Gonorrhoea, Syphilis.
- Viral STDs: HIV/AIDS, Genital Warts, Genital Herpes.
Question 9: What is menarche and menopause? [CBSE 2023] Answer:
- Menarche: The very first occurrence of menstruation in a young girl at puberty (around 10–12 years).
- Menopause: The permanent biological cessation of the menstrual cycle in women (usually occurring between ages 45–50).
Question 10: Why are the seeds produced by sexual reproduction not identical to parents? [CBSE 2024] Answer: Sexual reproduction involves the fusion of male and female gametes derived from two genetically distinct parent genomes, accompanied by crossing over during meiosis, producing new combinations of genes.
Question 11: What is the function of the prostate gland in the male reproductive system? [CBSE 2022] Answer: The prostate gland secretes a milky, slightly alkaline fluid that forms a major component of semen, nourishing sperms and neutralizing vaginal acidity to enhance sperm motility and survival.
Question 12: How does a Hydra reproduce by budding? [CBSE 2023] Answer: In Hydra, repeated cell division at one specific body location produces a small outward bud. As it grows, the bud develops small tentacles and a mouth, and upon full maturity, pinches off from the parent body to live independently.
Question 13: What happens when the egg is fertilized in the fallopian tube? [CBSE 2024] Answer: The fertilized egg forms a single-celled Zygote that divides rapidly as it travels down the oviduct to become a hollow ball of cells (Blastocyst), which implants itself into the thick, vascular uterine wall (Implantation).
Question 14: What is the main difference between fragmentation and regeneration? [CBSE 2024, 2020] Answer: Fragmentation is a natural asexual reproductive method occurring in simple organisms (Spirogyra) upon maturation; regeneration is an accidental healing/growth process in organisms like Planaria where severed body parts regenerate into whole individuals.
Question 15: Why is vegetative propagation considered an asexual mode of reproduction? [CBSE 2023] Answer: Vegetative propagation involves only a single parent plant without the formation of flowers, gametes, pollination, or fertilization, producing offspring that are genetically identical clones of the parent.
Mastering the NCERT Solutions for Class 10 Science Chapter 7, “How do Organisms Reproduce?”, equips students with the anatomical terminology, reproductive pathways, and developmental concepts required for the CBSE Board Examination. Review the male and female reproductive organ systems, the flower structure, and the 15 board-level FAQs above to secure full marks in your biology evaluations.